Find out surprising facts about how our minds do and don't retain information

There are many reasons why we forget at any age, though mix-ups are more common as you get older, says Barry Gordon, MD, PhD, founder of the Memory Clinic at Johns Hopkins Medical Institutions and coauthor of Intelligent Memory. Remembering is a three-step process: You acquire new info, consolidate it (a process in which the brain stores short-term memories more permanently, like saving a file in your computer's hard drive), and then later recall it. The first part is simple, but from there it gets trickier because every day we're bombarded by tons of new info.

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To prevent us from getting overwhelmed, the brain "edits" out things that it perceives as irrelevant, redundant or boring. In fact, without practice, more than 50 percent of new material often disappears from healthy minds within an hour.

Think of your brain as an email inbox that keeps overflowing with junk. If you didn't have a spam filter, you'd have a hard time getting to the most important messages. "There needs to be a balance of remembering and forgetting for memory to work properly," Dr. Gordon says. Most of the time, this goes smoothly, but sometimes your brain accidentally edits out important things because you're distracted or because the information is too similar to other information in your head. For example, remembering where you parked your car will be harder if you're busy ruminating about an argument. And because you've parked your car thousands of times before, you're also forcing your brain to remember where you parked today and forget all those other times.

Ever find yourself heading upstairs only to wonder why you went up there? Research suggests that we can only hold a maximum of seven thoughts in our mind at one time. "Some would argue we can only hold on to one," says Dr. Gordon. The task you intended to accomplish—say, grabbing a cell phone from your dresser—probably didn't feel more significant than whatever else was on your mind.

Categories & Connections

Another key reason we forget is that the brain likes to organize information by putting it into categories so that facts relate to each other. That's helpful in certain situations—such as when you remember the address 1776 4th Street because it makes you think of Independence Day—but it's also the reason why online passwords are prone to drift into the mental fog.

"You code them so nobody can figure them out, so you remove the associations that would normally jog your memory," says Mark Wheeler, PhD, associate professor of psychology in the Learning Research and Development Center at the University of Pittsburgh. Even if you use something familiar, like a pet's name, many sites require you to add a number, and remembering that you linked Fluffy's name with your daughter's birthday isn't always easy.

Proper names are also tough. Even trying to connect a name to a category can lead us down the wrong path. "When my mother occasionally calls her oldest grandchild by my name, she isn't forgetting who he is; she's thinking in categories, because I'm also an oldest child," says Dr. Gordon.

Lack of connection makes it especially difficult to remember an event in the future—what researchers call prospective memories—whether it's turning on the slow cooker, picking up your spouse at the airport or buying a birthday card. "It hasn't happened yet, so there's no link to an actual experience to fix it in your mind," says Dr. Wheeler.